Rewrite reStructuredText docstrings to use Google style

[seratch]

Summary

This pull request is preparation for #980 API document hosting.

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)
• /docs-src (Documents, have you run ./docs.sh?)
• /docs-src-v2 (Documents, have you run ./docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python setup.py validate after making the changes.

[seratch]

The slack package exists just for v2 backward compatibility (we will remove these in the long run). To reduce duplicated work for the future, let me delete these comments.

[codecov]

Codecov Report

Merging #981 (aca0401) into main (0ac8a0b) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #981      +/-   ##
==========================================
- Coverage   87.31%   87.30%   -0.02%     
==========================================
  Files          94       94              
  Lines        8541     8541              
==========================================
- Hits         7458     7457       -1     
- Misses       1083     1084       +1     

Impacted Files	Coverage Δ
slack_sdk/audit_logs/v1/async_client.py	88.75% <ø> (ø)
slack_sdk/audit_logs/v1/client.py	77.90% <ø> (ø)
...lation_store/async_cacheable_installation_store.py	70.27% <ø> (ø)
...installation_store/cacheable_installation_store.py	70.27% <ø> (ø)
slack_sdk/rtm/v2/__init__.py	51.88% <ø> (ø)
slack_sdk/scim/v1/async_client.py	93.13% <ø> (ø)
slack_sdk/scim/v1/client.py	90.47% <ø> (ø)
slack_sdk/socket_mode/builtin/internals.py	72.72% <ø> (-0.44%)	⬇️
slack_sdk/web/base_client.py	89.09% <ø> (ø)
slack_sdk/web/internal_utils.py	93.85% <ø> (ø)
... and 4 more

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